audio_devices_pref_handler: Add voice isolation state

The state will reflect the ui toggle state which will be added in
later CLs.

Setting kInputVoiceIsolationEnabled to allowlist in prefs_util.cc so
that HTML elements can access the pref.

BUG: b:353627012
TEST: autoninja -C out/Default chromeos_unittests && \
  out/Default/chromeos_unittests \
  --gtest_filter='*AudioDevicesPrefHandlerTest*'

Change-Id: Id9320a285847298749d7e8a6497eef620292bdbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5912313
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Li-Yu Yu <aaronyu@google.com>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Hung-Hsien Chen <hunghsienchen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1377091}